.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH INTRODUCTION
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The PCRE library is a set of functions that implement regular expression
pattern matching using the same syntax and semantics as Perl, with just a few
differences. The current implementation of PCRE (release 6.x) corresponds
approximately with Perl 5.8, including support for UTF-8 encoded strings and
Unicode general category properties. However, this support has to be explicitly
enabled; it is not the default.
.P
In addition to the Perl-compatible matching function, PCRE also contains an
alternative matching function that matches the same compiled patterns in a
different way. In certain circumstances, the alternative function has some
advantages. For a discussion of the two matching algorithms, see the
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\fBpcrematching\fP
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page.
.P
PCRE is written in C and released as a C library. A number of people have
written wrappers and interfaces of various kinds. In particular, Google Inc.
have provided a comprehensive C++ wrapper. This is now included as part of the
PCRE distribution. The
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\fBpcrecpp\fP
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page has details of this interface. Other people's contributions can be found
in the \fIContrib\fR directory at the primary FTP site, which is:
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.\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
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ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
.P
Details of exactly which Perl regular expression features are and are not
supported by PCRE are given in separate documents. See the
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\fBpcrepattern\fR
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and
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\fBpcrecompat\fR
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pages.
.P
Some features of PCRE can be included, excluded, or changed when the library is
built. The
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\fBpcre_config()\fR
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function makes it possible for a client to discover which features are
available. The features themselves are described in the
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\fBpcrebuild\fP
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page. Documentation about building PCRE for various operating systems can be
found in the \fBREADME\fP file in the source distribution.
.P
The library contains a number of undocumented internal functions and data
tables that are used by more than one of the exported external functions, but
which are not intended for use by external callers. Their names all begin with
"_pcre_", which hopefully will not provoke any name clashes. In some
environments, it is possible to control which external symbols are exported
when a shared library is built, and in these cases the undocumented symbols are
not exported.
.
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.SH "USER DOCUMENTATION"
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The user documentation for PCRE comprises a number of different sections. In
the "man" format, each of these is a separate "man page". In the HTML format,
each is a separate page, linked from the index page. In the plain text format,
all the sections are concatenated, for ease of searching. The sections are as
follows:
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  pcre              this document
  pcreapi           details of PCRE's native C API
  pcrebuild         options for building PCRE
  pcrecallout       details of the callout feature
  pcrecompat        discussion of Perl compatibility
  pcrecpp           details of the C++ wrapper
  pcregrep          description of the \fBpcregrep\fP command
  pcrematching      discussion of the two matching algorithms
  pcrepartial       details of the partial matching facility
.\" JOIN
  pcrepattern       syntax and semantics of supported
                      regular expressions
  pcreperform       discussion of performance issues
  pcreposix         the POSIX-compatible C API
  pcreprecompile    details of saving and re-using precompiled patterns
  pcresample        discussion of the sample program
  pcretest          description of the \fBpcretest\fP testing command
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In addition, in the "man" and HTML formats, there is a short page for each
C library function, listing its arguments and results.
.
.
.SH LIMITATIONS
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There are some size limitations in PCRE but it is hoped that they will never in
practice be relevant.
.P
The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
compiled with the default internal linkage size of 2. If you want to process
regular expressions that are truly enormous, you can compile PCRE with an
internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
distribution and the
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\fBpcrebuild\fP
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documentation for details). In these cases the limit is substantially larger.
However, the speed of execution will be slower.
.P
All values in repeating quantifiers must be less than 65536.
The maximum number of capturing subpatterns is 65535.
.P
There is no limit to the number of non-capturing subpatterns, but the maximum
depth of nesting of all kinds of parenthesized subpattern, including capturing
subpatterns, assertions, and other types of subpattern, is 200.
.P
The maximum length of a subject string is the largest positive number that an
integer variable can hold. However, when using the traditional matching
function, PCRE uses recursion to handle subpatterns and indefinite repetition.
This means that the available stack space may limit the size of a subject
string that can be processed by certain patterns.
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.\" HTML <a name="utf8support"></a>
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.SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
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From release 3.3, PCRE has had some support for character strings encoded in
the UTF-8 format. For release 4.0 this was greatly extended to cover most
common requirements, and in release 5.0 additional support for Unicode general
category properties was added.
.P
In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
the code, and, in addition, you must call
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\fBpcre_compile()\fP
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with the PCRE_UTF8 option flag. When you do this, both the pattern and any
subject strings that are matched against it are treated as UTF-8 strings
instead of just strings of bytes.
.P
If you compile PCRE with UTF-8 support, but do not use it at run time, the
library will be a bit bigger, but the additional run time overhead is limited
to testing the PCRE_UTF8 flag in several places, so should not be very large.
.P
If PCRE is built with Unicode character property support (which implies UTF-8
support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
The available properties that can be tested are limited to the general
category properties such as Lu for an upper case letter or Nd for a decimal
number. A full list is given in the
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\fBpcrepattern\fP
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documentation. The PCRE library is increased in size by about 90K when Unicode
property support is included.
.P
The following comments apply when PCRE is running in UTF-8 mode:
.P
1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
are checked for validity on entry to the relevant functions. If an invalid
UTF-8 string is passed, an error return is given. In some situations, you may
already know that your strings are valid, and therefore want to skip these
checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
at compile time or at run time, PCRE assumes that the pattern or subject it
is given (respectively) contains only valid UTF-8 codes. In this case, it does
not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
may crash.
.P
2. In a pattern, the escape sequence \ex{...}, where the contents of the braces
is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
code number is the given hexadecimal number, for example: \ex{1234}. If a
non-hexadecimal digit appears between the braces, the item is not recognized.
This escape sequence can be used either as a literal, or within a character
class.
.P
3. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8
character if the value is greater than 127.
.P
4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
bytes, for example: \ex{100}{3}.
.P
5. The dot metacharacter matches one UTF-8 character instead of a single byte.
.P
6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
but its use can lead to some strange effects. This facility is not available in
the alternative matching function, \fBpcre_dfa_exec()\fP.
.P
7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
test characters of any code value, but the characters that PCRE recognizes as
digits, spaces, or word characters remain the same set as before, all with
values less than 256. This remains true even when PCRE includes Unicode
property support, because to do otherwise would slow down PCRE in many common
cases. If you really want to test for a wider sense of, say, "digit", you
must use Unicode property tests such as \ep{Nd}.
.P
8. Similarly, characters that match the POSIX named character classes are all
low-valued characters.
.P
9. Case-insensitive matching applies only to characters whose values are less
than 128, unless PCRE is built with Unicode property support. Even when Unicode
property support is available, PCRE still uses its own character tables when
checking the case of low-valued characters, so as not to degrade performance.
The Unicode property information is used only for characters with higher
values.
.
.SH AUTHOR
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Philip Hazel
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University Computing Service,
.br
Cambridge CB2 3QG, England.
.P
Putting an actual email address here seems to have been a spam magnet, so I've
taken it away. If you want to email me, use my initial and surname, separated
by a dot, at the domain ucs.cam.ac.uk.
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Last updated: 07 March 2005
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Copyright (c) 1997-2005 University of Cambridge.
